Albert Barnes Commentary Psalms 21:10

"Their fruit wilt thou destroy from the earth, And their seed from among the children of men." — Psalms 21:10 (ASV)

Their fruit - Their offspring; their children; their posterity, for so the parallelism demands. The fruit is that which the tree produces; and hence, the word comes to be applied to children as the production of the parent. See this use of the word in (Genesis 30:2); (Exodus 21:22); (Deuteronomy 28:4), (Deuteronomy 28:11), (Deuteronomy 28:18); (Psalms 127:3); (Hosea 9:16); (Micah 6:7).

Shalt thou destroy from the earth - You shall utterly destroy them. This is in accordance with the statement so often made in the Scriptures, and with what so often occurs in fact, that the consequences of the sins of parents pass over to their posterity, and that they suffer in consequence of those sins. Compare (Exodus 20:5); (Exodus 34:7); (Leviticus 20:5); (Leviticus 26:39); compare the notes at (Romans 5:12–21).

And their seed - Their posterity.

From among the children of men - From among men, or the human family. That is, they would be entirely cut off from the earth. The truth taught here is, that the wicked will ultimately be destroyed, and that God will obtain a complete triumph over them, or that the kingdom of righteousness will eventually be completely established. A time will come when truth and justice will be triumphant, when all the wicked will be removed out of the way; when all that oppose God and his cause will be destroyed, and when God will show, by thus removing and punishing the wicked, that he is the Friend of all that is true, and good, and right.

The idea of the psalmist probably was that this would yet occur on the earth; the language is also such as may be applied to that ultimate state, in the future world, when all the wicked will be destroyed, and the righteous will no longer be troubled by them.